In an attempt to investigate the synergies between aluminum diethylphosphinate (AlPi) and phosphonate-based phosphorus-nitrogen (P-N) flame retardants (FRs) in glass fiber-reinforced poly(butylene terephthalate) (GF-PBT), novel piperazine analogue-based P-N FRs were synthesized. The thermal degradation and flame-retarding performance when cooperating with a commercial AlPi product, Exolit OP 1240, were determined, and their synergistic results were compared with a well-known melamine polyphosphate (MPP) synergist. As a result of TGA, SEM, and FTIR analyses, it was found that the phosphonate-based P-N FRs yielded improved synergism even with a lower P % content loading compared to MPP. To achieve a V-0 rating, the P content loading of piperazine analogue-based P-N FRs was 0.43% in the GF-PBT/OP1240 mixture at a 13.3 phr loading while that of MPP should be higher than 0.53% in the same formulation. The P content is the governing factor in the fire-retarding efficiency of phosphonate-based P-N FRs, which was well correlated with OP1240 in the gas phase. It is demonstrated that the combination of piperazine analogue-based P-N FRs with OP1240 significantly improves both thermal stability and flame retardancy of GF-PBT material.